All structures, both natural and man-made, are composed of materials that are arranged and assembled in a way that will fulfill the purpose of the structure. Buildings, bridges, and bones are made from clay, steel, and calcium. These have a correspondingly wide range of behaviors under load.
In planning a structure such as a bridge and assessing the usefulness of a particular material, the structural engineer must take into account many things--chiefly its cost, its weight, and above all, its strength. The strength of a material refers to its ability to withstand an applied load without failure. There are several types of load that can be applied--tension, compression, torsion, bending, and shearing.
The tensile, or tension, test is used to evaluate many of the properties of a material or a structure, such as the breaking load, the yield point, the ultimate tensile strength, the elongation under load, and so on.
The tensile test requires the use of a load frame; a structure that is able to clamp a specimen of the material between a stationary table and a moving crosshead. The load frame drives the crosshead thereby imparting a load to the specimen. The load is measured using a load cell, and any elongation of the specimen under load is measured using an extensometer.
Stress and Strain
When evaluating the properties of a material under test, the terms stress and strain are used to standardize the property values and to plot the behavior of the material using a stress/strain diagram.
Stress standardizes the load that is applied to the material by dividing the applied load by the initial cross-sectional area of the material.
Strain standardizes the elongation of the material under a specific load by dividing the change in length of the material by the original, or gauge, length.
Elastic and Plastic Strain and the Yield Point
It is generally accepted that many materials act like springs, meaning that they will stretch under load, but when the load is removed they return to their original length in similar fashion to stretching and releasing an elastic band.
This type of non-permanent deformation is called elastic strain. However, as the applied load increases, a point is reached where the material deforms to the extent that it cannot return to its original length because a permanent change has occurred in the material. The point at which this change takes place is called the yield point of the material. Strain occurring after the yield point is called plastic strain. Naturally enough, when designing a structure such as a bridge or a building, the structural engineer must select a material that will flex under load, but never reach the point at which the material will be permanently deformed.
Modulus of Elasticity (Young's Modulus) and Hooke's Law
In the elastic strain region, the relationship between stress and strain is linear. This relationship was described by the 17th century British physicist Robert Hooke and is known as Hooke's law.
The slope of the stress/strain line is called the modulus of elasticity or Young's modulus. The modulus of a material is a a constant and thus an indicator of its stiffness at low loads.
Ultimate Tensile Strength and Break Point
The tensile test indicates other important properties of a material. The ultimate tensile strength is the greatest load that the material experiences during the test. The break point is the point on the stress/strain curve at which the material breaks or ruptures. The load at the break point and the ultimate tensile strength are not necessarily the same; many materials will exhibit a reversal of the stress/strain curve after the maximum load point, where the load begins to fall away even as the extension is increasing.
Summary
The tensile test is an important tool for structural engineers who need to determine which materials to use in their projects. Its importance continues as new materials are designed to enable them to be used safely and with the confidence that comes with empirical testing.
Roger Tunsley - Roger has been a professional writer for over 25 years and is now enjoying the life of a freelance writer. Until August of 2009, Roger was ...
